1. Field of the Invention
The present disclosure relates generally to optics and, more specifically, to a system and method for the measurement of surfaces.
2. Description of the Related Art
The prior art can be said to go back to Hero of Alexandria (AD circa 10-70) who wrote up his observations about reflection. More recently, precision optics have been tested using the same law with the invention of the Foucault knife edge test in 1858 that is used to determine a profile of a mirror. More recently there are related tests such as the wire test, the Ronchi test that effectively uses a regular grid of wires to measure the slopes of surface in one direction at a time.
A two dimensional version of these slope measuring tests is the Hartmann test dating from 1900. With the availability of lens arrays and electronic cameras the modern version is the Shack-Hartmann test described in 1971.
A related technology called deflectometry was described by Ligtenberg in 1954 for measuring the deflections in plates, and that method has been improved over the years to the point where it can measure surface height variations at the nanometer (nm) level by measuring slopes as reported by Jueptner and Bothe in 2009. In these cases a pattern of stripes is programmed on the screen first horizontally and then vertically in order to measure the slope in both directions, something that is necessary to determine the complete surface topography. This paper describes using sinusoidally varying stripes in terms of intensity that can be phase shifted so the slope information can be obtained using the same phase shifting algorithm as used in interferometry. Peng (2010) describes another technique using square wave stripes rastered on the screen and capturing an image of each stripe and then finding the centroids of the crossing points in computer processing of the data to find the slope in both directions. He applied the method to the testing of large astronomical mirrors to the tens of nm level.
In addition to having to measure the slope in two orthogonal directions, there must be information in the data about how far the surface is from the screen and camera. Haeusler, U.S. Pat. No. 7,532,333, points out that there are an infinite number of surfaces that can be fit through a series of measured slopes. His patent describes using two cameras and triangulation to determine the needed distance to establish the precise surface being measured. In precision optics this distance can be determined by independent means so only one camera and screen are necessary to determine the surface.
As mentioned above, the programmable metrology screen technology may be applied to refractive systems as well. U.S. Pat. No. 6,616,279 gives an example of another type of technology where the refractive system is the human eye and a reflective device is used to interrogate the light refracted by the eye.